Standard generators used in catheter ablation procedures provide radio frequency (RF) energy in a unipolar fashion between one or more electrodes supported on an ablation catheter and a ground electrode applied to the patient. The delivery of ablation energy is controlled by monitoring rises in the tissue-electrode interface temperature or tissue impedance.
Recent bench studies comparing a standard generator delivering energy simultaneously to multiple electrodes to delivering pulsed RF (PRF) energy to multiple electrodes shows that pulsing produces contiguous lesions more consistently. Conventional PRF energy delivery systems deliver packets of energy to multiple electrodes at a set frequency. Once an electrode reaches a specified temperature, excess pulsed energy is diverted to a shunt resistor. Studies suggest, however, that the convective heat loss using PRF at the electrode-tissue contact point is faster than the heat conduction within the myocardium. As a result, the peak tissue temperature achieved using PRF energy can occur at depths of approximately 2 mm below the electrode within the myocardium rather than at the electrode-tissue interface. The extent of convective heat loss into the blood pool that circulates about the indwelling ablation catheter will vary with the quality and amount of electrode-tissue contact. When a portion of the ablation electrode surface area is not in contact with tissue ("poor tissue contact"), that portion of the electrode will be exposed to the circulating blood pool, resulting in the temperature sensor on the catheter reading lower temperatures for that electrode than if a greater portion of the electrode were in good contact with the tissue. A conventional system response to the low temperature reading is the application of further PRF energy to that electrode in an attempt to reach and maintain a temperature set point.
In practice, as the temperature at an electrode reaches about 100.degree. C., a sharp impedance rise is detectable as the blood begins to boil and the denatured plasma proteins begin to adhere to that electrode. To counter this occurrence, most generators offer an adjustable impedance cut-off point that will shut down the generator if it detects such an impedance rise (typically, a rise from about 80-100 .OMEGA. to about 150-200 .OMEGA.). The risk of having peak tissue temperature reached within the myocardium instead of at the electrode-tissue interface is that super heating of the myocardium can occur and go undetected (i.e., a greater temperature may exist within the body organ than was measured at the electrode/tissue interface). If such a condition is not detected or thwarted, an explosion effect can occur within the myocardium causing extensive tissue damage long before a peak temperature or impedance rise is detected.
What is needed in the art is a system and method for detecting poor tissue contact conditions. What is further needed is such a system and method that can use such data to control pulsed radio frequency energy delivered to a tissue site. The present invention satisfies these and other needs.